Evolution models of helium white dwarf-main-sequence star merger remnants: The mass distribution of single low-mass white dwarfs

Abstract

It is not known howsinglewhite dwarfs with masses less than 0.5M⊙ - low-masswhite dwarfs - are formed. One way in which such a white dwarf might be formed is after the merger of a helium-core white dwarf with a main-sequence star that produces a red giant branch star and fails to ignite helium. We use a stellar evolution code to compute models of the remnants of these mergers and find a relation between the pre-merger masses and the final white dwarf mass. Combining our results with a model population, we predict that the mass distribution of single low-mass white dwarfs formed through this channel spans the range 0.37-0.5M⊙ and peaks between 0.45 and 0.46M⊙. Helium white dwarf-main-sequence star mergers can also lead to the formation of single helium white dwarfs with masses up to 0.51M⊙. In our model, the Galactic formation rate of single low-mass white dwarfs through this channel is about 8.7 × 10-3 yr-1. Comparing our models with observations, we find that the majority of single low-mass white dwarfs (≤0.5M⊙) are formed from helium white dwarf-main-sequence star mergers, at a rate which is about 2 per cent of the total white dwarf formation rate.